In prior art silica-loaded tire compounds, bis(triethoxysilylpropyl) tetrasulfide is often used as a coupling agent for forming a bond between rubber and silica. This compound, however, has the problem that when milled with rubber and silica at elevated temperatures, it causes the blend to increase its Mooney viscosity to such an extent as to restrain subsequent working.
Then JP-A 8-259739 discloses a method for improving the workability of a rubber compound loaded with silica by blending bis(triethoxysilylpropyl) disulfide therein. When bis(triethoxysilylpropyl) disulfide is used alone, however, workability is improved at the sacrifice of low fuel consumption characteristic of the silica-loaded tires.
We thus paid attention to polysulfide silane mixtures and discovered that a good compromise is made between low fuel consumption and workability when a mixture of polysulfide silanes represented by the following general formula (1) is used. EQU (RO).sub.3 SiC.sub.3 H.sub.6 S.sub.n C.sub.3 H.sub.6 Si(OR).sub.3(1)
Herein n is a positive number having a distribution whose average is in the range: 2.1.ltoreq.n.ltoreq.2.9, and R is methyl or ethyl.
With respect to the preparation of polysulfide silanes, JP-A 7-228588 discloses a method involving reacting Na.sub.2 S with sulfur to produce polysulfides and, without isolating the polysulfides, continuously reacting the reaction product with a halogenoalkoxysilane. JP-A 7-228588, however, addresses the preparation of tetrasulfide silanes. No reference is made to short-chain polysulfide silanes of formula (1) wherein n is as defined above which can endow excellent properties to silica-loaded rubber tire compounds.
Another known method for preparing short-chain polysulfide silanes is by starting with tetrasulfide silanes and effecting desulfurization using sodium cyanide. Undesirably, sodium cyanide is toxic and the method requires an increased cost.
Short-chain polysulfide silanes can also be prepared by reacting pure sodium disulfide or sodium trisulfide with halogenoalkoxysilanes as disclosed in JP-A 47-29726. The preparation of pure sodium disulfide or sodium trisulfide requires a substantial cost, and consequently, the silane product becomes expensive.